This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to an EL driver circuit that can be implemented in relatively small die.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder.
The prior art discloses several types of drivers including an inductive boost circuit having an inductor in series with a switching transistor. Current through the inductor causes energy to be stored in a magnetic field around the inductor. When the current is abruptly shut off, the induced magnetic field collapses, producing a pulse of high voltage. The voltage across the inductor is proportional to Lxc2x7di/dt. Thus, a low voltage at high current is converted into a high voltage at low current. The voltage on the lamp is pumped up by a series of high frequency, high voltage pulses from the boost circuit.
The direct current produced by the boost must be converted into a low frequency (200-2000 Hz) alternating current in order to power an EL lamp. It is known in the art to switch either the inductor or the EL lamp in such a way as to produce alternating current through the lamp.
It is known in the art to provide a driver for an EL lamp with a boost inverter, a capacitor for storing high voltage, and an H-bridge output coupled to the storage capacitor; see U.S. Pat. No. 5,463,283 (Sanderson). It is also disclosed in this patent to use constant current sources on the high side of the H-bridge. One constant current source is used for each half of the bridge.
It is known in the art to use a series connected resistor and transistor as a discharge circuit; see U.S. Pat. No. 5,982,105 (Masters). It is known that an EL lamp can produce acoustic noise due to the abrupt discharge of the lamp when polarity is reversed. The abrupt discharge also causes a current spike to flow through the lamp that shortens the life of the lamp. It is also known to control the discharge current through an EL lamp to minimize noise generated by the lamp; e.g. U.S. Pat. No. 5,789,870 (Remson) and U.S. Pat. No. 6,038,153 (Andersson et al.). It is also known to discharge an EL lamp before charging the lamp; see the Sanderson patent and U.S. Pat. No. 6,087,863 (Aflatouni).
Commercially available drivers use a pair of controlled current devices for discharging a lamp for a minimum period of time, thereby reducing the noise generated by an EL lamp. The power spikes associated with discharging an EL lamp severely stress the discharge devices. As a result, the devices are robustly made, making the semiconductor die on which the driver is implemented too large for many small packages, e.g. what is known as an MSOP (Mini Small Outline Plastic) package, which is one fourth the size of an SOP package. An MSOP package is approximately three millimeters by five millimeters in size.
It is known in the art to locate a switching transistor on a separate die to reduce die size; e.g. see U.S. Pat. No. 5,780,975 (Krafcik). In this patent, it is disclosed that one of the switching transistors in a boost circuit is separate from the die incorporating the remainder of the driver.
In view of the foregoing, it is therefore an object of the invention to minimize the size of the on-chip discharge devices in a driver for an EL lamp.
Another object of the invention is to provide a high power EL driver in a very small semiconductor package.
A further object of the invention is to provide an EL driver in an MSOP package.
Another object of the invention is to eliminate one of the discharge devices, thereby further reducing the size of the die.
Another object of the invention is to provide an external path for lamp current, thereby still further reducing the size of the die implementing the driver.
The foregoing objects are achieved in this invention in which a driver for an EL lamp includes a voltage boost circuit and an H-bridge output having an AC diagonal and a DC diagonal. An EL lamp is coupled across the AC diagonal. In accordance with one aspect of the invention, a current path is coupled in series with the DC diagonal and limits current through the EL lamp. In accordance with another aspect of the invention, the current path is external to the semiconductor device implementing the driver.